THE MIDCONTINENT RIFT
SYSTEM IN IOWA
LOWER RED CLASTICS
Introduction to Lower Red Clastics Group
The rocks encountered at depth between 3153 and 5310 m
(10,510 to 17,700 ft) in the Eischeid well were informally
named the Lower Red Clastic Group. The group was divided into
three component units by Witzke (1990), from the top, Unit D,
Unit C, and Unit B. To view a discussion of each of the
induvidual units that comprise the Upper Red Clastics Group,
click on the appropriate name on the following chart.
LOWER RED CLASTICS GROUP STRATIGRAPHY
MIDCONTINENT RIFT SYSTEM ROCKS OF IOWA
(click for map of
thickness of Lower Red Clastics in flanking basins)
Discussion of the Lower Red Clastic Group.
The Lower Red Clastic Group closely resembles and may
correlate with the Oronto Group in the Lake Superior area.
The biggest differences between the two groups are the
coarser-grained facies preserved in the Copper Harbor
Conglomerate, the basal unit in the Oronto Group, and the
dominance of quartz sand in Unit B, the basal unit of the
Lower Red Clastic Group. The Copper Harbor is dominated by a
conglomeratic facies composed primarily of mafic and felsic
volcanic rock clasts. Associated sandstone facies also are
dominated by volcanic rock fragments. Unit B is dominated by
a sandstone facies that displays the highest quartzose grain
content (87%) of any Red Clastic unit in the Eischeid well
(click here for QFL information). This
disparity can be explained by the positions of the two units
relative to the central horst of the Midcontinent Rift. The
Copper Harbor Conglomerate exposures are located on the
central horst, which was an axial graben at the time of
Copper Harbor deposition. The volcanic-dominated rocks of the
Copper Harbor Conglomerate were deposited by alluvial fans
that were derived in large part from the erosion of
Keweenawan volcanic rocks capping the footwalls of the
graben-bounding normal faults. In contrast, the Eischeid
well, and Unit B, are located in one of the clastic basins
that flank the central horst, and Unit B was therefore
deposited outside of the central graben. Unit B is dominated
by quartzose sediment derived from granitic rocks, the most
common basement rock lithology in the MRS region, and was
deposited by rivers that flowed towards the axis of the rift.
As these rivers approached the axial graben they may have cut
through footwall-capping volcanics, depositing Copper Harbor
lithologies in the graben.
Unit C is lithologically and sedimentologically very
similar to the Nonesuch Formation of the Oronto Group. The
fine grained components of Unit C display unidirectional,
bi-directional, symmetrical, and trough cross stratification
and small scale amalgamated hummocky cross stratification.
Shrinkage cracks are also present in this interval (Ludvigson
and others, 1990). Milavec (1986) identified similar features
in the Nonesuch Formation.
The Nonesuch Formation has been interpreted as a lake
deposit by most workers (e.g., Elmore, 1981; Daniels, 1982;
Milavec, 1986). However, some workers have suggested that it
may have been deposited in a marine environment (Burnieand
others, 1972; Hieshima and others, 1989; and Hieshima and
Pratt, 1991). In either case, the Nonesuch appears to have
been deposited in a standing body of water. Structures
observed in cores of Unit C also suggest deposition in a
standing body of water (Ludvigson and others, 1990). The
presence of these lake deposits in the Eischeid well indicate
that the body of water was not confined to the central horst,
the position where correlative Nonesuch deposits are
observed. Interpretation of petroleum industry seismic data
suggests that Unit C may extend at least 11 km (7 mi) west of
the Iowa Horst in the Defiance Basin, and perhaps as much as
24 km (15 mi) or more.
Unit C cores show significant deformation that is not
present in cores of other intervals. The entire interval of
Core #4 (4813 to 4817 m; 16,043 to 16,058 ft) displays a high
angle of tectonic dip, ranging from 65 degrees to vertical or
slightly overturned (Ludvigson and others, 1990).
Additionally, Ludvigson and Spry (1990) reported a
silckensided brittle fault surface in this interval, with a
set of rough facets resulting from secondary mineral growth
during faulting, indicative of reverse faulting. They also
identified smaller pull-apart microfaults/veinlets displaying
a reverse sense of displacement and folds with white calcite
veins on the outer circumference of the fold hinge which they
interpreted as filled extension fractures.
The black coloration in Unit C, like the Nonesuch
Formation, is primarily the product of disseminated organic
carbon, along with minor pyrite. Palacas and others (1990)
reported total organic carbon (TOC) values as high as 1.4%
(averaging 0.6%) in the dark shales and siltstones from cores
of Unit C. The Nonesuch Formation has yielded maximum TOC
values of almost 4% (Hieshima et al., 1989). The petroleum
source rock potential of Unit C is discussed later.
Unit D occupies the same stratigraphic position in the
Eischeid well as the Freda Formation in the Oronto Group of
the Lake Superior area. Interpretation of Core #1 from Unit D
suggested (Ludvigson and others 1990) that the unit was
deposited in a fluvial environment. A similar interpretation
has been proposed for the Freda Formation by many workers
(e.g., Daniels, 1982; Morey and Ojakangas, 1982b).
Petrographic studies of Unit C revealed a mean detrital
composition of Q-76 F-21 L-3 compared to a mean composition
of Q-52 F-18 L-30 for seven Freda exposures reported by
Daniels (1982). The lithic component of the Freda is
dominated (72%) by volcanic rock fragments. The higher
volcanic lithic component indicates that volcanic rocks were
exposed in the Freda source area, and similar lithologies
were not exposed in the source area of Unit D.
click for references cited